The strongest statement by Jesus in regard to the use of this
characterization of one person by another is in Matt. 5:22, where
mōros reflects
the Aramaic word raca, which means incapable of life. This section is not
dealing with what specific titles one can or cannot call another person, but
with a supposed believer’s attitude toward others, especially covenant brothers.

The Greek term, mōros, translated "fool," was meant
to reflect the Aramaic term raca. However, Jesus’
word play was not to the Greek word mōros, but the
primarily Hebrew word mōreh, BDB 598, which meant
"rebel against God" (cf. Num. 20:10; Deut. 21:18,20; see F. F. Bruce,
Answers to Questions, p. 42). Jesus called the
Pharisees by this very term in Matt. 23:17. Not only our actions, but our
motives, attitudes, and purposes determine sin against our fellow human. Murder,
as far as God is concerned, can be a thought! Hatred of our brother or sister
clearly shows that we do not know God (cf. 1 John. 2:9-11; 3:15, and 4:20).
Socially speaking, a hateful thought is better than a death, but remember that
this section of Scripture is meant to shatter all self-righteousness and pride
in one’s own goodness.

Paul follows the OT and Jesus in using several terms to describe foolish
people.

1. aphrōn, 1 Cor. 15:36; 2 Cor. 11:16,19; 12:6,11

2. mōros, 1 Cor. 3:18; 4:10 and a related form in Rom. 1:22

People who claim to know God, but think and act in inappropriate ways, are often
characterized as poor thinkers! Paul's sarcastic comments, so frequent in
1 and
2 Corinthians, reveal this type of person. They were so confident that they
possessed knowledge that they could not see nor recognize true knowledge!